Ball point writing instrument



July 29, 1969 R. L. DOUGLAS 3,458,255

7 BALL POINT WRITING INSTRUMENT Filed Sept. 15, 1966 "(Q U 2! 3 24 MIKE lllllllllllllllllll u w 20 42 25 lit;- 5 50 INVENTOR ROBERT L. DOUGLAS United States Patent 3,458,266 BALL POINT WRITING INSTRUMENT Robert L. Douglas, Janesville, Wis., assignor to The Parker Pen Company, Janesville, Wis., 21 corporation of Wisconsin Filed Sept. 15, 1966, Ser. No. 579,750 Int. Cl. B43k 7/02 US. Cl. 401209 7 Claims ABSTRACT OF THE DISCLOSURE The inner surfaces of the ink reservoir have an energy level of no more than 20 dynes/centimeter and are provided along their entire length with closely spaced annular grooves or steps that cooperate with the ink to deter the tipping of the meniscus into an unstable condition. The inner surfaces of the walls may comprise a fluoropolymer of the monomer 1H, lH-pentadecafluorooctyl methacrylate or of the monomer Z-(N-propylperfiuorooctane sulfonamido) ethyl acrylate.

The present invention relates in general to ink reservoirs for ball point writing instruments, and it relates more particularly to ink reservoirs which are connected at one end to the ball socket and open to the atmosphere at the other end.

This type of reservoir may be of the so-called capillary type wherein the meniscus at the rear end of the ink column is sufficiently strong to maintain its stability and prevent the ink from moving rearwardly in the reservoir, or it may employ means other than the meniscus for this same purpose. The latter types of reservoirs have included such things as rigid or non-rigid followers disposed at the rear end of the ink column or very flexible, deformable sacks which apply atmospheric pressure to the ink column while maintaining the continuity thereof.

All of these prior art ink reservoir systems have certain advantages and disadvantages, and it would be desirable to provide a reservoir which would obviate the need for any type of follower or sack in a reservoir having an internal diameter substantially greater than that of the heretofore known capillary reservoirs. Such a reservoir would have the advantage of simplicity now associated with capillary reservoirs and it would also have the advantage of a large ink capacity now associated with the follower type reservoirs.

In order to prevent the ink from flowing out of the rear end of a non-capillary reservoir when the pen is disposed in an inverted position, it has been suggested in United States Patent 2,732,829 to provide one or more peripheral or annular continuous steps or grooves on the inner surfaces of the reservoir between the rear end of the ink column and the open end of the reservoir. This grooved surface is said to function as a barrier to prevent the flow of ink from the reservoir, but since it does not provide for meniscus stability it does not prevent the trapping of air within the ink column. As is well known in this art, when air bubbles which are trapped in the extremely viscous ink reach the narrow feed channel connected between the larger diameter reservoir tube and the ball socket, the supply of ink to the ball is interrupted and the pen stops writing.

It is, therefore, an object of this invention to provide a reservoir for ball pen ink which enables a stable meniscus in a tubular reservoir having an internal diameter of the order of .2 inch.

Another object of this invention is to provide a ball pen reservoir having a low energy ink engaging surface which cooperates with the ink to provide a stable meniscus which will not tip into an unstable condition when the pen is inverted.

A further object of this invention is to provide a reservoir configuration which retards meniscus tip throughout the length of the reservoir.

A still further object of this invention is to configure a low energy ink engaging reservoir surface to provide sufiicient stability for the meniscus to prevent the ink from leaking out of the rear end of the reservoir and to prevent the trapping of air in the ink column.

Briefly, the above and further objects ar'e realized in accordance with one aspect of the present invention by providing a tubular reservoir open at one end and closed by a ball at the other end with an ink engaging surface having an energy level of no more than about 20 dynes/ centimeter. In accordance with another aspect of this invention the ink engaging surface of the reservoir is provided throughout its length with closely spaced annular grooves or steps which cooperate with the ink to deter the tipping of the meniscus into an unstable condition. The combination of the grooves and the low energy surface provides a particularly satisfactory reservoir when the internal diameter is as high as about .2 inch.

Further objects and advantages and a better understanding of the present invention may be had from a reading of the following detailed description and claims with reference to the accompanying drawings wherein:

FIGURE 1 is a generally schematic sectional view taken along the longitudinal center line of a ball point p FIGURE 2 is a sectional view of a ball point pen cartridge embodying the present invention;

FIGURE 3 is an enlarged view of a portion of the reservoir of FIGURE 2; and

FIGURES 4, 5, 6 and 7 are enlarged sectional views of portions of other embodiments of the present invention.

In order to provide a stable meniscus in a tubular reservoir having an internal diameter of about .175 inch or less when using present-day ball pen inks having a glycol base such as propylene glycol, it is necessary that the reservoir wall have an effective surface energy level no greater than 20 dynes/cm. Where a larger internal diameter is desired, the surface energy of the wall must be lower. For example, a surface energy level of about 11 dynes/cm. or less is necessary for the satisfactory operation of a smooth-walled reservoir having an internal diameter of .213 inch. Only recently have any materials been available which meet these energy level requirements and which are physically suited for use as the walls of ball pen reservoirs.

It has been found, however, that the ability of the reservoir wall to deter migration of the ink with a consequent lessening of the tendency for the meniscus to distort when the pen is inverted can be improved by the proper design of the wall surface whereby the wall surface has a predetermined configuration. Accordingly, by the proper selection of the shape or configuration of the wall surface and the material from which it is formed, relatively large diameter reservoirs which do not require the use of followers or sacks of any type may be provided.

Before consideringthe particular materials and shapes which may be used to provide a satisfactory reservoir, a brief discussion of the hydraulics involved in the functioning of an open end ball pen reservoir may be helpful. Referring to FIG. 1, there is shown, in schematic form, a ball point pen disposed in a horizontal plane with the open end of the reservoir on the left. The meniscus at the open end of the ink column, when viewed in cross-section along the central longitudinal axis thereof, is in the shape of a reversed S. As long as the net moment of the forces acting about the center line C-C is zero or in a counterclockwise direction, the meniscus is stable and maintains the ink column in place. If, on the other hand, the net moment is in a clockwise direction, the meniscus is unstable and the ink column will move toward the open end of the reservoir at the left until a stable condition is reached or the ink leaks out of the reservoir. It can be mathematically determined that the greater the difference between the advancing contact angle and the receding contact angle 0 the larger the diameter of the reservoir which can be used without causing an unstable meniscus. In the art, the difference between the advancing and receding contact angles (6 0 is sometimes referred to as the hysteresis of contact angle. Inasmuch as the characteristics of a given ink are not generally controllable, the hysteresis of contact angle can only be controlled by the proper selection of the shape of the wall of the reservoir and the material from which it is made. Since the hysteresis of contact angle between a liquid and a solid surface is roughly inversely proportional to the energy of the solid surface, the allowable reservoir diameter increases with a decrease in the surface energy of the reservoir wall. The energy of the solid surface is generally expressed in the art in terms of dynes/centimeter and is sometimes called the critical surface tension.

Most present-day ball pen inks have a propylene glycol base, a viscosity at 25 degrees C. between 6500 and 14,000 centipoises, a surface tension between 32 and 44.5 dynes/ cm., and a specific gravity at 25 degrees C. between 1.00 and 1.17. When using such inks in a tubular, cylindrical reservoir having a smooth inner wall, the wall surface must have an energy level or critical surface tension no greater than 20 dynes/cm. to provide a reservoir of sufficient capacity to be marketed in competition with the follower type reservoirs. Tetrafluoroethylene, known and marketed by DuPont under the name Teflon has a critical surface tension of about 18 dynes/cm. and may be satisfactorily used as the smooth wall surface of a reservoir having an internal diameter of .186 inch or less. A fiuoro-polymer of the monomer 1H, lH-pentadecafluorooctyl methacrylate, having the chemical formula available as a 2% solution by weight in hexafluoroxylene and sold by Minnesota Mining and Manufacturing Company of St. Paul, Minn. under the designation FX-706, or of the monomer 2-(N-propylperfiuorooctane sulfonamido) ethyl acrylate, having the chemical formula may be satisfactorily used as the smooth wall surface of a reservoir having an internal diameter of .213 inch or less.

Another factor which affects the ability of the meniscus to maintain itself in a stable condition is the presence of impurities or high energy surfaces on the wall of the reservoir in the vicinity of the meniscus. The principal source of such impurities is the ink itself which in some cases leaves a residue on the reservoir wall as the level of ink diminishes during use of the pen. It has been found, however, that there is essentially no ink residue deposited on the reservoir wall when the surface energy level is less than 20 dynes/cm. Accordingly, the ability of a low energy surface reservoir to resist meniscus tip does not change as the ink supply is depleted as is the case with high energy surfaces.

In order to further increase the ability of the reservoir and the ink to maintain a stable meniscus and/or to increase the internal diameter of the reservoir, there is provided in accordance with this invention several different reservoir wall surface configurations which restrict the tendency of the meniscus to tip or move into an unstable condition. While these wall surface configurations may be used successfully with surface energy levels exceeding 20 dynes/cm. it is preferred to employ these configurations with surface energy levels of less than 20 dynes/cm.

Referring now to FIG. 2, there is shown a cross-sectional view of a ball point pen employing an ink reservoir constructed in accordance with one aspect of this invention. As there shown, a ball pen 20 includes a tip section 22 including a feed tube 21 and a ball and socket assembly 23 consisting of a writing ball 24 rotatably mounted in a socket. The feed tube 21 includes a narrow feed channel or bore which connects the socket to the principal portion of the reservoir. The entire tip section 22 is preferably metal and a plastic reservoir tube 26 is sealably fitted thereover such that the space within the tube directly communicates with the bore in the tip section. Thus, ink may feed from the tubular section 26, through the bore of the feed tube to the socket and the ball.

Throughout the effective length of the tube 26, the internal wall is provided with annular steps 28 which appear in cross-section as saw tooths or serrations. The most forward portion of the inner wall of the tube 26 may be smooth to facilitate the sealing of the tube 26 to the tip section 22. Similarly, the most rearward portions of the inner wall may be smooth to receive a closure member or protective cap. If expedient for purposes of manufacture, however, the tube 26 may be provided throughout its entire length with the steps 28.

Referring to FIG. 3, it may be seen that each of the steps 28 consists of a gently tapered portion which meets with a generally radial wall portion 32 along a circular line 34. Accordingly, the line 34 forms a sharp edge or discontinuity which inhibits the flow of ink from the tapered portion 30 to the radial portion 32. It may thus be seen that the tube 26 is preferably oriented with the step portions 30 tapering out toward the point of the pen so as to provide the maximum resistance to meniscus tip. Also, it is preferable that the annular area 35 which interconnects adjacent ones of the steps 28 be beveled to minimize the collection of ink residue which would adversely affect the ability of the steps to retard meniscus tip. The portion 30 of each step should be made very smooth and in some cases may require buffing for this purpose.

The tube 26 may be molded of high density polyethylene or of some other shape-retaining plastic which is sufficiently resilient to permit facile withdrawal of a mandrel or core pin having the step portions thereon. Following the molding operation, it is preferred that the inner wall of the tube 26 be coated with the fluoropolymer FX-706 described hereinbefore or with some other low energy material. Although the entire tube may be molded of a low surface energy material, the high cost of such material would, at the present time, be prohibitive. Thereafter the tube 26 may be sealed over the end of the tip section 22 and the resulting cartridge may then be filled with ink in the conventional manner.

While the surface configuration shown in FIGS. 2. and 3 is believed to be satisfactory from both an operational and a manufacturing point of view, other shapes or configurations may be better for certain applications. In FIG. 4, for example, there is shown a wall surface configuration which provides a substantially greater discontinuity across each of the steps 38. In this case each of the steps 38 consists of two annular sections 40 and 42 which are both tapered outwardly in the direction of the ball point and meet along an annular line 43 to provide a sharp edge. This configuration finds application where the meniscus has a greater tendency to tip in the reservoir as, for example, where the reservoir wall has a higher energy level or a larger internal diameter is desired. The configuration of FIG. 4, however, is more difficult to manufacture than is that of FIGS. 2 and 3.

In FIG. 5 there is shown another surface configuration for providing edge or discontinuity between each step 44 in the wall of the reservoir. While this configuration is also more resistant to meniscus tip than is that of FIGS. 2 and 3, it is more ditficult and costly to manufacture.

The embodiment of FIG. 6 provides a lesser resistance to meniscus tip than does any of the other illustrated configurations but it has the advantage of being less expensive to make. Accordingly, where the reservoir diameter is sufliciently small and/or where the surface energy level is sufficiently low, the surface configuration of FIG. 6 may be used. As there shown, the reservoir wall is formed by convergent and divergent wall portions 58 and 60 which meet along annular lines 62 and 64. The line 62 provides the necessary L-shape, continuous edge to deter any undue tipping of the meniscus.

As explained hereinbefore, the tube 26 may be formed of a low energy surface material or it may be formed of a high energy surface material and later coated with a low energy material such as FX-706. The following method of applying the coating of FX-706 to the inner wall o the cartridge has proven to be satisfactory.

After a tube 26 formed of polyethylene has been removed from the mold, it is press-fitted and cemented onto the end of the feed tube portion of the writing tip 22. At this stage of the operation the ball and socket assembly 23 has not yet been aifixed to the feed tube 21. The assembly of the tube 26 and feed tube 21 is then filled with FX-706, which, at room temperature, is a very thin liquid, and the FX 706 is immediately allowed to drain out through the feed tube 21. The assembly is then placed in a standing up position in an oven at about 140 degrees F. for about one hour to drive off the solvent (hexafluoroxylene). Thereafter, the assembly is removed from the oven, allowed to cool, and the ball tip 22 is secured to the feed tube 21. As a result of this process, an extremely thin, transparent layer of a low surface energy material is provided on the internal walls of the reservoir tube 26 and the feed tube 21. The cartridge may then be filled with ball pen ink in the known manner.

The susceptibility of ball pen inks to air and moisture varies from one type and color to another but it is generally desirable to minimize the flow of air across the surface or meniscus of the ink contained in the reservoir. To this end, it is desirable to locate a plug in the rear end of the reservoir and to provide an extremely narrow passageway therethrough. The passageway thus equalizes the pressure behind the ink column with that of the ambient atmosphere but it retards the transfer of air into and out of the reservoir.

One difliculty with the use of a narrow passageway is the possibility of its becoming clogged by foreign matter. This can be readily avoided, however, in several ways. For example, the plug can be provided with several passageways or it may be made of a porous material. Where the associated pen is of the propel-repel type, a single longitudinal hole through the plug may be aligned with a cleaning pin on the propel-repel mechanism so that the pin is inserted into the hole and withdrawn therefrom each time the mechanism is operated.

Another way in which the transfer of moisture to the ink has been minimized while maintaining atmospheric pressure on the ink column is shown in FIG. 7. As there shown, a longitudinal passageway or bore 50 extends through an otherwise imperforate plug 52 secured in the rear end of the tube 26. A counterbore 54 is provided at the forward end of the plug 52 and a porous, spongelike annular member 56 is disposed therein. The member 56 is impregnated with a liquid having a high afiinity for water whereby the air in the rear portion of the reservoir is maintained in a relatively dry state. It has been found that propylene glycol, a standard ink component, works well for this purpose.

While the present invention has been described in connection with particular embodiments thereof, it will be understood that many changes and modifications may be made by those skilled in the art without departing from the invention. Therefore, by the appended cleams, it is intended to cover all such changes and modifications as come within the true spirit and scope of the invention.

I claim:

1. A ball point pen, comprising a tubular shape retaining member holding therein a supply of ball pen ink and having a front end and a rear end, and

a ball point writing assembly connected to said front end of said member in direct communication with said ink for writing out the ink contained in said member,

said rear end of said member being connected to the ambient atmosphere so that the space therein rearward of said ink is the ambient pressure,

the entire ink engaging surface of said tubular member having an energy level of no more than twenty dynes per centimeter, and

wherein said ink engaging surface is formed of a polymer of a monomer of the group consisting of 1H,-H- pentadecafluorooctyl methacrylate and Z-(N-propylperfluorooctane sulfonamido) ethyl acrylate. whereby said ink forms a column defined in part by the inner surface of said tubular member and remains in communication with said ball point writing assembly irrespective of the orientation of said pen.

2. A ball point pen according to claim 1 wherein said surface is serrated.

3. A ball point pen according to claim 2 wherein said serrations are defined by tapered wall portions and transverse wall portions.

4. A ball point pen according to claim 3 wherein said tapered wall portions diverge toward the forward end of the member.

5. A ball point pen according to claim 2 wherein said serrations are each defined by first and second wall portions which respectively diverge toward the forward end of the tubular member.

6. A ball point pen according to claim 1 further comprising a porous perforated member positioned in the rear end of said tubular retaining member, said porous member being impregnated with a liquid having a high afiinity for water, whereby the air in the rear portion of said reservoir is maintained in a relatively dry state.

7. A ball point pen reservoir comprising a tubular member having an inner surface whose energy level is no greater than 20 dynes/centimeter.

said inner surface being provided with spaced apart annular grooves to inhibit the tilting of the meniscus at the surface of a column of ink contained in said member, and

wherein said surface is a thin low surface energy film of a polymer of a monomer of 1H,1H-pentadecafluorooctyl methacrylate.

References Cited UNITED STATES PATENTS 2,222,599 11/ 1940 Ashmore 401-25 3 2,732,829 1/ 1956 Fehling 401-217 3,030,925 4/ 1962 Dyson 401209 FOREIGN PATENTS 834,821 3/ 1952 Germany. 525,326 3/1955 Italy.

LAWRENCE CHARLES, Primary Examiner 

